[W/K] :: regular expression

  regular expression
       
          1.  (regexp, RE) One of the wild
          card patterns used by Unix utilities such as grep, sed
          and awk and editors such as vi and Emacs.  These use
          conventions similar to but more elaborate than those described
          under glob.  A regular expression is a sequence of
          characters with the following meanings:
       
          An ordinary character (not one of the special characters
          discussed below) matches that character.
       
          A backslash (\) followed by any special character matches the
          special character itself.  The special characters are:
       
          "." matches any character except NEWLINE; "RE*" (where
          the "*" is called the "{Kleene star") matches zero
          or more occurrences of RE.  If there is any choice, the
          longest leftmost matching string is chosen, in most
          regexp flavours.
       
          "^" at the beginning of an RE matches the start of a line and
          "$" at the end of an RE matches the end of a line.
       
          [string] matches any one character in that string.  If the
          first character of the string is a "^" it matches
          any character (except NEWLINE, in most regexp flavours)
          and the remaining characters in the string.  "-" may be used
          to indicate a range of consecutive ASCII characters.
       
          \( RE \) matches whatever RE matches and \n, where n is a
          digit, matches whatever was matched by the RE between the nth
          \( and its corresponding \) earlier in the same RE.  In
          many flavours ( RE ) is used instead of \( RE \)
       
          The concatenation of REs is a RE that matches the
          concatenation of the strings matched by each RE.
       
          \< matches the beginning of a word and \> matches the end of a
          word.  In many flavours of regexp, \> and \< are replaced by
          "\b", the special character for "word boundary".
       
          RE\{m\ matches m occurences of RE.  RE\{m,\} matches m or
          more occurences of RE.  RE\{m,n\ matches between m and n
          occurences.
       
          The exact details of how regexp will work in a given
          application vary greatly from flavour to flavour.  A comprehensive
          survey of regexp flavours is found in Friedl 1997 (see below).
       
          [Jeffrey E.F. Friedl, "{Mastering Regular
          Expressions(http://enterprise.ic.gc.ca/~jfriedl/regex/index.html),
          O'Reilly, 1997.]
       
          2. Any description of a pattern composed from combinations
          of symbols and the three operators:
       
          Concatenation - pattern A concatenated with B matches a match
          for A followed by a match for B.
       
          Or - pattern A-or-B matches either a match for A or a match
          for B.
       
          Closure - zero or more matches for a pattern.
       
          The earliest form of regular expressions (and the term itself)
          were invented by mathematician Stephen Cole Kleene in the
          mid-1950s, as a notation to easily manipulate "regular sets",
          formal descriptions of the behaviour of finite state
          machines, in regular algebra.
       
          [S.C. Kleene, "Representation of events in nerve nets and
          finite automata", 1956, Automata Studies. Princeton].
       
          [J.H. Conway, "Regular algebra and finite machines", 1971, Eds
          Chapman & Hall].
       
          [Sedgewick, "Algorithms in C", page 294].
       
          (1997-08-03)